package com.benben.springSecurity.weiwei;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class PhoneNumber {
    Map<String,String> phone = new HashMap<String, String>(){{
        put("2","abc");
        put("3","def");
        put("4","ghi");
        put("5","jkl");
        put("6","mno");
        put("7","pqrs");
        put("8","tuv");
        put("9","wxyz");
    }};
    List<String> output = new ArrayList<String>();
    public void backtrack(String combination,String next_digits){
        if(next_digits.length()==0){
            output.add(combination);
        } else {
            String digital = next_digits.substring(0,1);
            String letters = phone.get(digital);
            for(int i=0;i<letters.length();i++){
                String letter = letters.substring(i,i+1);
                backtrack(combination+letter,next_digits.substring(1));
            }
        }

    }
    public List<String> letterCombinations(String digits){
        if(digits.length() != 0) {
            backtrack("",digits);
        }
        return output;
    }
}
